School of Engineering and Science - Jacobs University

DISCUSSION
limiting conditions, nutrients are added in excess to the dilution series and unfertilized
bottles serve as control for the calculation of the natural phytoplankton growth rate
(Landry, 1993). Sometimes the feeding response of the microzooplankton is not linear
due to food saturation at lower dilution levels. In these cases, additional results from
higher dilution levels at which feeding is still linear can be applied to estimate the
grazing rate (Gallegos, 1989, Gallegos et al., 1996).
While recognizing the restrictions of the dilution technique, it still has the fundamental
advantage of barely altering natural prey and grazer communities and only excluding
larger zooplankton. Thus, natural interactions within the plankton community are
included in dilution experiments. This technique is now standard for assessment of in
situ grazing rates of microzooplankton and was also used during this study (see Chapter
III).
One principal restriction of dilution experiments was addressed during this thesis:
Several microzooplankton species are highly fragile and sensitive to handling. Filling
and mixing procedures during experiments (Gifford, 1985, Landry, 1993, Broglio et al.,
2003) can cause considerable losses in those species. Based on this fragility nondestructive
methods have been developed to prevent the loss of sensitive species.
However, these methods which were meant to handle microzooplankton with greater
care have never truly been evaluated in an experimental set-up.
Results presented in Chapter II show that even techniques previously considered
conservative for microzooplankton species can have significant negative impacts on
their abundance and diversity. Handling procedures are always necessary during
laboratory experiments and especially dilution experiments require several treatment
steps (e.g. screening, preparation of the dilution series, filling of incubation bottles). I
showed that the consequences of such manipulation of water samples while setting up
grazing experiments can significantly alter the grazer community through the loss of
sensitive taxa. This defeats the goal of a grazing experiment aimed at the determination
of the in situ grazing rate, which can indeed only be measured when the natural in situ
grazer community is present in an experiment. The consequence of such alteration is
even worse when the degree of bias in the community is unknown. My results show that
it is imperative to monitor potential effects of handling procedures on the community
under study. Such evaluation is crucial not only for the extrapolation of experimental
results to the field, but also for the understanding of processes within the investigated
system. It futhermore leads to the advancement of present techniques and further
innovations.
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